1. Field of the Invention
This invention relates to a system for controlling fuel metering in an internal combustion engine, more particularly to a system for controlling fuel metering in an internal combustion engine wherein the quantity of fuel injection is optimally determined over the entire range of engine operating conditions including transient engine operating condition using an intake air model and by simplifying its calculation.
2. Description of the Prior Art
In a conventional fuel metering control system, the quantity of fuel injection was usually determined by retrieving mapped data predetermined through experimentation and stored in advance in a microcomputer memory using parameters having intrinsically high degrees of correlation with the quantity of air drawn in the engine cylinder. As a result, the conventional technique was utterly powerless to cope with any change in the parameters which had not been taken into account at the time of preparing the mapped data. Further, since the mapped data were intrinsically prepared solely focussing on the steady-state engine operating condition and the transient engine operating condition was not accounted for, the conventional technique was unable to determine the quantity of fuel injection under the transient engine operating condition with accuracy. For that reason, there are recently proposed techniques to establish a fluid dynamic model describing the behavior of the air intake system so as to accurately estimate the quantity of air drawn in the cylinder such as disclosed in Japanese Laid-Open Patent Application 2(1990)-157,451 or U.S. Pat. No. 4,446,523.
Similarly the assignee proposed in Japanese Patent Application 4(1992)-200,330 (filed in the United States on Jul. 2, 1993 under the number of 08/085,157) a method for estimating the quantity of air drawn in the cylinder by determining the quantity of throttle-past air while treating the throttle (valve) as an orifice to establish a fluid dynamic model based on the standard orifice equation for compressible fluid flow. The fluid dynamic model used was, however, premised on an ideal state and required various assumptions. It was therefore impossible to wipe out all the errors which could be introduced at the time of modeling. Further, since it was quite difficult to accurately determine constants such as the specific-heat ratio used in the model, errors possibly arising therefrom could disadvantageously be accumulated. Furthermore, the equation necessitated calculation of powers, roots or the like. Since approximate values were used for them in practice, additional errors resulted.
The assignee therefore proposed in Japanese Patent Applications 4(1992)-306,086 and in the additional application claiming the domestic priority thereof (5(1993)-186,850)(both filed in the United States on Oct. 18, 1993 under the number of 08/137,344 and patented under the number of U.S. Pat. No. 5,349,933) a system for controlling fuel metering in an internal combustion engine which, although it was based on a fluid dynamic model, could absorb errors in the model equations and optimally determine the quantity of fuel injection over the entire range of engine operating conditions including the transient engine operating condition without conducting complicated calculations. In addition, the assignee proposed an improvement of the technique in Japanese Patent Application 5(1993)-208,835 (filed in the United States and patented as above). Specifically, as illustrated in FIG. 10, a large quantity of air passes through the throttle valve at a time when it was opened, since the pressure difference across the throttle plate was large at the transient engine operating condition. In the improved technique, therefore, the assignee proposed to describe the quantity of throttle-past air at the transient engine operating condition by calculating a ratio (referred to as "RATIO-A") between the effective throttle opening area A and its first-order lag value ADELAY, so as to absorb errors in model equations and optimally determine the quantity of fuel injection irrespective of the operating condition of the engine or presence/absence of aging of the engine.
However, as illustrated in FIG. 22, the TDC interval, i.e., the control or program (calculation) interval (cycle) varies with the engine speed. The interval (cycle) at a low engine speed (shown as "INT-L" in the figure) becomes longer than that at a high engine speed (shown as "INT-H" in the figure). As a result, as will be apparent from FIG. 23A, the ratio (RATIO-A=A/ADELAY) becomes excessively large at a low engine speed so that the ratio is not always appropriate for describing the quantity of throttle-past air at the transient engine operating condition illustrated in FIG. 23B (which is similar to that shown at the bottom of FIG. 10).